1. Field of the Invention
The present invention relates to a method and an apparatus adapted to measure an amount by which an object to be measured has moved in plane and back-and-forth in non-contacting fashion at a high speed without a demand for particular index or mark substantially freely from conditions of said object such as temperature, color or material thereof.
Irradiation of the object to be measured with monochromatic laser beam having high directivity, high luminance and linearity generates a granular speck pattern upon its reflection. This granular speck pattern depends on a degree of roughness on the surface of the object irradiated with the laser beam. The granular speck pattern translates in proportion to expansion or extension as the object is heated or an external stress is exerted on the object. On the other hand, the granular speck pattern is reduced or magnified as the object moves back-and-forth.
The present invention intends to pick up this granular speck pattern in direct and optical manner as an index, to calculate a movement of the object with respect to said index and to display the calculated result as the amount of movement.
2. Description of the Related Art
A non-contact method for such measurement is well known in which the object is irradiated with laser beam vertically to the object and there are provided a pair of unidimensional sensors at an angle of 45° about the normal line, respectively.
Irradiation of the object with laser beam results in generation of the granular speck pattern due to interference of scattering light. Output of the respective unidimensional sensors is used as reference signal to observe movement of the granular speck pattern as the surface of the object is moved or distorted. Such movement is photoelectrically detected at two points and a differential movement between these two points is obtained to automatically eliminate a rigid body movement component. This is so-called cross correlating method used to determine a distortion.
According to another method of prior art, a darkroom is used as the measurement environment and a granular speck pattern generated by the reflecting laser beam is projected on a screen in the form of frosted glass. This projected pattern is picked up by a digital camera or the like, then a granular speck is selectively extracted by a computer and amount of movement of the object in plane is determined on the basis of an amount by which a granular point thereof has moved.
A high accuracy is required for the optical parts as well as the optical system mechanism in the case of the method as has been described above for calculating an amount of distortion or movement from interference fringes.
In addition, when a correlative peak value and a reference signal value determined by the cross correlating method is lower than a threshold value, processing is inevitably complicated and becomes troublesome. The measurement environment in the form of a darkroom is essential for projection on the screen formed by frosted glass.